1. Technical Field
This invention relates to the art of automatically changing machine tools and, more particularly, to method of determining when rotary tools are sufficiently worn to require changing.
2. Description of the Prior Art
A crucial aspect for future factory automation will involve development of systems for automatic tool changing to maximize productivity while maintaining high levels of product quality. An ideal tool monitoring system would activate a tool changer when the cutting force reached a level corresponding to the end of the useful life of the tool, but prior to any catastrophic failure of the tool.
Prior known attempts to achieve this goal have included the use of audible sound spectra, acoustic emission intensity, temperature and touch probes, power consumption, and vibrations. Each of these sensing criteria have failed to provide an adequate system because (i) descernible signals, responsive to wear, occur too close to impending breakage, (ii) require impractical sensing equipment, and (iii) fall far short of being consistently reliable.
In particular, Beer ("Power Consumption--A Measure of Tool Performance", Soc. of Mfg. Engrs., Technical Paper MR79-398, Dearborn, Mich., 1979), disclosed a system employing power consumption as an indicator of wear. It is difficult to isolate consumption due to wear only, and this requires complex accounting for power losses not due to chip formation or wear. Bloomquist ("Sensors for Batch Manufacturing of Metal Parts", Proceedings of the American Society for Metals International Conference on High Productivity Machining, Materials and Processes", New Orleans, La., May 7-9, 1985) discloses a vibration based method for detecting dull tools. Such method is undesirable because it required attachment of a complex accelerometer and could not detect significant vibrations until the tool was substantially close to breakage; the utility of this method is further reduced by the normal use of bushings in the tool holder which inherently muffle vibrations.
Two authors have investigated the sensing of force related parameters. Colwell ("Analytical Strategies for Automatic Tracking of Tool Wear", Proceedings of the Sixth North American Manufacturing Research Conference, Gainesville, Fla., Apr. 16-19, 1978, pp. 276-282) used a system of cummulative percent deviation of chip thickness and cutting forces to render a more informed signal of worn lathe tools. Such work did not investigate the difficulty of sensing worn rotary tools and failed to recognize the use of threshold force values set just below steady-state force levels of tools in the abrasive wear regime.
Fabris and Prodder ("Optimization of Drill Life"Influence of Cutting Conditions on Tool Wear", Proceedings of the Tenth North American Research Conference, Hamilton, Ontario, Canada, May 24-25, 1982, pp. 337-341) tracked the rate of change of thrust or torque and required complex interpretation which is not sufficiently responsive to consistently prevent tool failures. The author specifically taught to avoid direct use of force parameters.